Flame photometric detectors are now in common use as relatively inexpensive spectroscopic detectors for use with gas chromatographs, particularly for the analysis of sulfur and phosphorus substances in the effluent from the gas chromatograph. As described in an article entitled "Gas Chromatographic Detectors" by C. H. Hartmann in Analytical Chemistry, Vol. 43, No. 2, pages 113A to 125A, February 1971, the basic elements of the flame photometric detector include a burner jet where the sample of effluent from the GC is burned or heated in the combustion provided by H.sub.2 fuel gas and a combustion supporting gas such as oxygen or air, and a spectrophotometer for observing the sample including a suitable light filter and photodetector. When substances containing sulfur are brought into contact with the hydrogen rich flame, they emit a characteristic light spectrum at about 360-410 nm; phosphorus-containing substances produce light emission centered about 526 nm.
In the known form of such detectors, as illustrated by the structures shown in U.S. Pat. No. 3,290,118 issued Dec. 6, 1966 to C. Van Der Smissen entitled "Apparatus For Detecting Phosphorus And/Or Sulfur In Gases" and U.S. Pat. No. 3,489,498 issued Jan. 13, 1970 to S. Brody et al. entitled "Flame Photometric Detector With Improved Specificity To Sulfur And Phosphorus", the sample under test, for example air or the column effluent from a gas chromatograph mixed with the combustion supporting gas such as oxygen or air, is delivered to the burner tip through a first tube or passageway. The hydrogen is delivered to the burner housing via a separate passageway where it burns with the sample gas and oxygen and produces the desired combustion.
The flame produced by such a device may not be reliable in operation since, for example, it is subject to blowout resulting from a sudden surge of solvent in the effluent from the chromatograph and a resultant oxygen starvation. In addition, should the column output including the oxygen source be disconnected from the detector input, the flame is extinguished and the hydrogen may escape and create a safety hazard.
Additionally, when the sample and carrier gas are fed into the very hot center portion of the hydrogen and oxygen flame, the burning of substances such as organic compounds which may be present results in an interfering light emission, reducing the specificity of the detector, i.e., the ability to respond primarily to a select group or groups of substances with a minimal response to all other substances. Special efforts have been employed in the past in an effort to block this interfering light emission from the spectrophotometer; for example, in U.S. Pat. No. 3,489,498 a cylindrical shield is provided about the tip of the burner so that the lower portion of the flame where such interference-producing burning normally takes place is not in the optical line of sight of the photospectrometer so that such interfering light emission will not degrade the selectivity. However, blocking the emitted light in this fashion tends to reduce detector sensitivity, i.e., the effectiveness of the detector as a transducer in converting the sample into a measurable electrical signal, since it also blocks a portion of the desired emission.